1. Field of the Invention
This invention relates to a HDTV that carries out correction of error and channel equalization at the same time in a vestigial side band transmission system.
2. Discussion of the Related Art
The Grand-Alliance vestigial side band (VSB) digital transmission system has a transmitter for receiving data input, for converting the received data input into a transmission format, and for transmitting the converted data input through a transmission channel. A receiver is provided for converting the data received from the transmitter through a transmission channel into a reception format and transmitting the converted data.
The VSB system transmits data according to the data frame of a VSB transmission system as shown in FIG. 1.
A frame consists of segments which have 836 symbols each, and each of the segments includes four symbols of segment synchronization signal, 832 symbols of data and a FEC (Forward Error Correction) signal.
Each data frame has 313 segments, one of which is a data field synchronization segment containing training sequence and 312 segments are general data segments.
The training sequence signal is a signal contained in the field synchronization signal, transmitted from the transmitter to the receiver, for correcting errors caused when the data is transmitted through a transmission channel at the receiver. The training sequence signals in odd fields are identical and the training sequence signals in even fields are inverted signals of the training sequence signals in the odd fields.
The channel equalizer is a system element used in the receiver for compensation of linear channel distortion, such as tilt and ghost, caused by defective system elements that are in a transmission channel or a receiver.
Channel equalization is a process that uses a filter, opposite to the transmission characteristic of a channel, for reducing channel distortion caused during transmission, wherein the most important question is how to calculate the filter coefficient.
For example, when data of `I` is transmitted from a VSB transmission system, in case the characteristic function of the transmission channel is `f`, the receiver receives a signal of `I.multidot.f`.
The channel equalizer in the receiver is provided with an inverted function of f.sup.-1, which is an inversion of the characteristic function of the transmission channel, for making the input signal become I.multidot.f.multidot.f.sup.-1 =I once it has passed the channel equalizer, thereby restoring the input signal to be the original input signal.
Thus, the channel equalizer, obtaining an inverted function which is an inversion of the characteristic function of the transmission channel, restores a signal transmitted through a transmission channel to an original signal without any distortion.
The filter coefficient of the channel equalizer can be obtained with the LMS (Least Mean Square) algorithm. This LMS algorithm can be expressed in the following equation. EQU C.sub.k+1 =C.sub.k +.DELTA.(I.sub.TK -I.sub.Tk)I.sub.in equation ( 1),
wherein, C.sub.k+1 is the filter coefficient of the channel equalizer, C.sub.k is a filter coefficient obtained previously, I.sub.Tk1 is a training sequence signal already decided at the transmitter, I.sub.Tk2 is a training sequence signal of a channel equalizer output, containing errors due to transmission through a transmission channel, and I.sub.in is the input signal of the channel equalizer.
As shown in FIG. 2, the conventional channel equalizer includes a finite impulse response filter (hereinafter called `FIR filter`) 1 for receiving input signal I.sub.in and filter coefficient C.sub.F, a slicer for converting the input signal into a signal having a predetermined transmission level, a filter coefficient calculator 5 for receiving a synchronization signal SYNC, the training sequence signal applied from outside (i.e., applied from the receiver) I.sub.Tk1, and the training sequence signal which was converted into a signal having a predetermined transmission level I.sub.Tk2 through the slicer 3, and generating filter coefficients C.sub.F and C.sub.I based on the received signals. An infinite impulse response filter (hereinafter called `IIR filter`) 4 receives the filter coefficient C.sub.I transmitted from the filter coefficient calculator 5 and the converted training sequence signal I.sub.Tk2. An adder 2 receives signals from the FIR filter 1 and the IIR filter 4, adds the-two signals and transmits the added signal as an output signal I.sub.out of the equalizer.
As has been explained, the conventional equalizer has two filters of FIR filter 1 and IIR filter 4, wherein the FIR filter 1 carries out filtering of the input signal Iin utilizing the filter coefficient C.sub.F applied from the filter coefficient calculator 5, and the IIR filter 4 carries out filtering of the training sequence signal converted into a signal having a predetermined transmission level I.sub.TK2 through the slicer 3 utilizing the filter coefficient C.sub.I applied from the filter coefficient calculator 5. The adder 2 adds the signals received from the FIR filter 1 and the IIR filter 4 and transmits the result of the addition as the output signal of the channel equalizer I.sub.out.
The signal received from the adder 2 I.sub.out is not filtered by directly applying the received signal to the IIR filter 4, but is instead filtered by applying the received signal fed back after conversion into a signal having a predetermined transmission level through the slicer 3 to the IIR filter 4. The reason for not directly filtering is that the channel equalizer cannot converge but rather diverges the received signal, in case channel equalization is carried out, by applying the training sequence signal, which contains errors, to the channel equalizer.
The filter coefficient calculator 5 performs the filter coefficient calculation using the input signal I.sub.in, the training sequence signal applied by the receiver I.sub.Tk1, the converted training sequence signal having a predetermined transmission level I.sub.Tk.sub.2, and the synchronization signal SYNC.
The training sequence signal I.sub.Tk1 and the synchronization signal SYNC are signals applied from outside of the channel equalizer, and the synchronization signal SYNC indicates the position of the received training sequence signal I.sub.Tk1.
Since the training signal I.sub.Tk1 is a value already determined at the transmitter, it always has a specific value, whereas the converted training sequence signal having a predetermined transmission level I.sub.Tk2 varies depending on the error value.
Therefore, the filter coefficient calculator 5 calculates error value (I.sub.Tk1 -ITk.sub.2) based on the training sequence signal I.sub.Tk1 received from the slicer 3 and the training sequence signal I.sub.Tk1 applied from outside the transmitter during the training sequence period in which the synchronization signal SYNC is being applied.
Accordingly, the constant .DELTA. is multiplied by the error value (I.sub.Tk1 -I.sub.Tk2), and by the input signal I.sub.in.
Upon addition of this multiplied value to the previous filter coefficient C.sub.k, the filter coefficient shown in equation (1) can be obtained.
Then, the FIR filter 1 and the IIR 4 filter can carry out channel equalization using the filter coefficients C.sub.F and C.sub.I received from the filter coefficient calculator 5.
However, the conventional channel equalizer is only operable during a training sequence period, and because the channel equalizer has a significantly low converging speed, the error correction speed is slow, and cannot correct the phase errors contained in the transmission signals because the channel equalizer carries out channel equalization only using I signals, but not Q signals as the input signals.